The story
From custom (custom car); the usage of a "K" for Kustom rather than a "C", is believed to have originated with George Barris.
Etymology from Wiktionary, CC BY-SA 4.0.
Kustom
Today it means: A uniquely modified, post-World War II American automobile built or styled between the 1930s and early 1960s; rather than aiming for performance like early "hot rods", these cars were heavily stylized for show, featuring chopped roofs, molded body lines, and radical paint jobs. Full definition →